In a medical field, there is known a technology of carrying out inspections, diagnoses, and the like by irradiating excitation light onto a living body and measuring fluorescence generated by the excitation. Patent Literature 1 discloses a fluorescent observing device that uses a semiconductor laser as a light source for excitation light. In this fluorescent observing device, excitation light emitted from the semiconductor laser is irradiated onto a body tissue from a tip end portion of an endoscope. Then, fluorescence generated from the body tissue by the excitation light is photographed by an imaging element and displayed on a display unit.
As shown in FIG. 1 of Patent Literature 1, in the semiconductor laser, a Peltier device and a thermistor are arranged so as to be in close contact with each other. By the Peltier device being driven on the basis of a temperature detected by the thermistor, the temperature of the semiconductor laser is constantly kept at 20° C. or less. By adjusting the temperature of the semiconductor laser in this way, an output of excitation light is increased, and a lifetime is prolonged (paragraphs [0014], [0044], [0045], etc. in specification of Patent Literature 1).